1. Field of the Invention
The present invention relates to a crystal growth apparatus and more specifically to a crystal growth apparatus for growing a crystal by unidirectionally solidifying a melt contained in a crucible by using a vertical Bridgman furnace in a cylindrical and vertical magnetic field space.
2. Description of the Prior Art
There has conventionally been used a method for growing a crystal in a crystal growth furnace incorporated into a strong magnetic field-generating apparatus as a means for growing a crystal in a strong magnetic field. However, taking into consideration the magnitude of a magnetic field applied, size of an effective magnetic field space, the kinds of heat sources and the effect of a magnetic field on driving mechanism (motor), the application of the crystal growth furnace is considerably limited. For instance, when making use of a cylindrical narrow vertical magnetic field space, the most suitable crystal growth furnace is a vertical Bridgman furnace because of the shape of the effective magnetic field space and the simplicity of the installation of a crystal growth apparatus and the simplified operations thereof.
Upon assemblage of a crystal growth apparatus wherein a vertical Bridgman furnace is arranged within a cylindrical vertical magnetic field space such as that disclosed in Journal of Crystal Growth, 1983, 62, pp. 207-209, a driving mechanism for rising and lowering a combustion tube which accommodates a crucible at a predetermined position is constructed independently of a heat-generating furnace and the combustion tube and arranged within the space under a vertical magnetic field-generating apparatus (hereinafter referred to as "water-cooled magnet") and in the proximity of the extension of the central axis (hereinafter referred to as "axis of magnetic field") of the cylindrical magnetic field space. On the other hand, the heat-generating furnace and the combustion tube are inserted into and placed in the cylindrical and vertical magnetic field space through the upper part of the water-cooled magnet and the lower end of the combustion tube passes through the water-cooled magnet and fitted to the foregoing driving mechanism within the space under the water-cooled magnet. Such a structure permits the rising and lowering of the combustion tube.
In the conventional crystal growth apparatus having the foregoing structure, the driving mechanism is constructed independently of the heat-generating furnace and the combustion tube as discussed above. Accordingly, the operation for fitting the lower end of the combustion tube to the driving mechanism must accurately be carried out within a relatively narrow space under the water-cooled magnet and the position of the driving mechanism relative to the heat-generating furnace and the combustion tube must be precisely adjusted in such a manner that the central axis of the combustion tube coincides with the axis of the magnetic field, each time the vertical Bridgman furnace is arranged within the cylindrical and vertical magnetic field space. However, it is quite troublesome to accurately arrange and position the combustion tube within such a relatively narrow space under the water-cooled magnet. This impairs the workability and if the alignment of these apparatuses is incomplete, the combustion tube comes in contact with the inner wall of the heat-generating furnace during rising and lowering operations and causes vibrations. This leads to agitation of the melt in the crucible and accordingly interferes with the optimum crystal growth.
Moreover, in the conventional crystal growth apparatus having the foregoing structure, the evacuation or replacement of gases in the combustion tube is carried out at the lower end of the combustion tube. This makes the structure of the lower part of the tube complicated and correspondingly makes the assemblage thereof quite difficult. In the conventional crystal growth apparatus having the foregoing structure, the driving mechanism must be miniaturized and positioned in the proximity of the extension of the axis of the magnetic field applied and at a position in the close proximity to the water-cooled magnet (for instance, a position about 45 cm distant from the center of the magnetic field) in order to make effective use of the limited space under the water-cooled magnet. If a relatively strong magnetic field of, for instance, not less than 5 T (tesla) is applied to the apparatus having the foregoing structure, the motor as a driving source is affected by the magnetic field. This results in unstable rotation thereof and interferes with the stable rising and lowering of the combustion tube at a constant speed. For this reason, the magnitude of the magnetic field applied must be limited to a relatively low level on the order of not more than, for instance, 4 T.